Casey made a desperate call to police as she collapsed after an unprovoked attack by a woman. She was wounded once in the stomach.

The teenager dialled 999 but managed only a brief conversation with the operator before collapsing near a play area.

A tennis coach described on Wednesday how he had tried to save her.

“I’ve had first aid training and suggested we put her in the recovery position,” said John Willis, 58.

“She was on the ground lying on a coat. A woman who had also seen her was stroking her hair, trying to reassure her that she’d be all right.

“She was having difficulty breathing and only moved a fraction, I think to get comfortable. She was very pale and never said anything.”

He later laid flowers at the park, with a message reading: “We found you, we tried to help, we didn’t know you but we loved you. God bless, the Willis family.”

Casey had left her home in Rossington, South Yorkshire, at midday on Tuesday to catch the bus five miles into Doncaster. She was in or near Elmfield Park, close to the town centre, when the woman, who is known to social services, launched her attack at 1.15pm.

Schools in the area are currently on half-term holiday.

Police found two knives nearby. Detectives were trying to piece together the movements of both the girl and her attacker, who may have been on the bus with her.

Mr Willis, who had been playing on nearby tennis courts with his sons Owen, 10, and Matthew, five, and one of their friends, said the children had been taking a break in the play area when they dashed back to him.

“They said, 'There’s a lady having a fit. She’s on the ground. Can you come and help, Daddy?’

“So I ran over as quickly as I could. I found this lady who I now know to be a girl of 13 on the ground. There were two other passers-by attending to her already. One of them I’d seen earlier with her children. She was telling the girl there were people around, helping her and keeping her calm.”

He said that a police officer arrived and cleared the girl’s airway. A paramedic followed minutes later and began trying to resuscitate her. Casey was taken to Doncaster Royal Infirmary but died on Tuesday night.

The teenager lived with her mother, Kelly Day and stepfather, Mark Day. Her natural father is Anthony Kearney, a mechanic. All were too upset to speak on Wednesday.

Det Supt Terry Mann, leading the investigation, said that the attack had been “random” and “isolated”.

He said: “We know that Casey-Lyanne Kearney left her home in Rossington around 12 noon on Wednesday and caught the number 55 bus into town to meet a friend.

“At 1.18pm Casey called police from inside Elmfield Park saying she’d been stabbed. She was treated at the scene by police and paramedics before being taken by ambulance to Doncaster Royal Infirmary. Sadly, she passed away later that night.”

He said that Casey, who was 5ft 3in with red hair, had been wearing a black waist-length jacket and blue jeans and carrying a black Bench shoulder bag with purple writing on it.

Jo Miller, Doncaster council’s chief executive, said: “The person in custody was known to a number of local public agencies and we are working together to establish all the circumstances involved with this case.”

The park lies close to the town centre, within yards of Doncaster Register Office, and police were scouring CCTV footage from the office on Wednesday.

One woman, who was passing with her 13-year-old daughter, said that she no longer allowed her child to go into the park. “It is not a very good park, to be honest. There have been a number of incidents in there,” she said.

Her daughter had come across a drunk woman close to the park on Tuesday afternoon, she added.

The woman’s daughter said: “We were just walking and we decided to run.

“She was walking past and she asked us why we are running. She started calling us names.”

One local shopkeeper, who gave his name as Raja, said: “They should put CCTV cameras in the park. A few customers have been saying that.”

A 26-year-old woman named Hannah Bonser was charged with the schoolgirl’s murder on Wednesday night, police said.

In 1991 Emma Brodie, 11, was stabbed to death in the nearby Frenchgate Centre by a woman with psychiatric problems. Carol Ann Barratt later pleaded guilty to manslaughter on the grounds of diminished responsibility.